Such interferometer systems use two light beams coming from the same source, generally a laser type diode, and propagating in opposite directions along the same optical path which is constituted by a monomode optical fiber, prior to interferring on a photodetector. They serve to show up non-reciprocal phase changes which depend on the direction of beam propagation and which are due to the Sagnac effect in a gyroscope or to the Faraday effect when measuring a current, and this takes place preferentially compared with reciprocal changes in phase which are due to various disturbances encountered by the beams as they propagate around the interferometer ring and which cancel by compensation at the photodetector.
When special dispositions are not taken, the optical power of the two beams at the photodetector gives rise to a D.C. component and to a component which is proportional to the cosine of their relative phase difference, and this suffers from the drawback of imparting to the photodetector a cosine detection law with zero sensitivity at the origin. In order to eliminate this drawback, it is conventional to apply periodic and symmetrical phase modulation to the two beams running round the interferometer ring, said modulation having a period 2.tau. of twice the time .tau. taken by the beams to run around the interferometer ring. As described in published French patent application number FR-A-2 471 583, such phase modulation gives rise, at the photodetector, to a signal having a component of frequency 1/2.tau. which is proportional to the sine of the relative phase difference between the two beams, thereby giving rise to maximum sensitivity at the origin over a range of small relative phase differences.
In addition to using phase modulation of the beams from which they are constituted, Sagnac type interferometer systems employed in gyroscope applications or for measuring current by the Faraday effect generally make use of an integrated optical Y-coupler for separating the source light wave into two beams prior to applying it to the optical fiber constituting the interferometer ring, and also serving to recombine the two output beams from the interferometer ring, together with a directional coupler for separating the light wave from the source and the two return beams from the interferometer ring.
Such integrated optical devices are preferred over conventional optical solutions because of their advantages of insensitivity to vibration, compactness, . . . , but they never-the-less suffer from the drawback of having non-negligible absorption coefficients and of being relatively difficult to implement. That is why proposals have already been made, in particular in French patent applications number FR-A-2 582 800 to omit the directional coupler by combining the light source and the photodetector in such a manner as to make them colinear, either by making both of them in the form of a single semi-conductor diode which is used alternatively as an emitter and as a detector of light energy, or else by aligning both of them, with the light source being constituted by a semiconductor diode which is coupled via both its front face and its rear face and which is interposed between the photodetector and the Y-coupler, and diode being used alternatively as an emitter and as an amplifier of light energy.
In either case, the semiconductor diode is switched so as periodically to emit light pulses which are as long as possible, i.e. of duration just less than the time .tau.' required for the two beams to propagate over their entire go-and-return paths. The switching period 2.tau.' is then very close to the period 2.tau. of the phase modulation used for optimizing detection sensitivity since the go-and-return path length covered by the beams between the light source and the interferometer loop is small compared with the path link of the interferometer loop itself. As a result, in the output signal from the photodetector, the various components due to the modulation coming from source switching and coming from phase modulation overlap one another, thereby making it difficult to detect the useful signal.
In said French patent specification FR-A-2 582 800, this difficulty is averted by artificially doubling the propagation time of the two beams on their go-and-return paths by adding an additional length of optical fiber between the source and the Y-coupler, said additional length being equal to one fourth of the length of the interferometer loop.
The object of the present invention is to solve the above-mentioned problem without adding a significant length of optical fiber in order to avoid increasing the bulk of the inter-ferometer system and to avoid reducing light energy efficiency.